It’s the longest recorded migration of any mammal, stretching from 10,000 to 12,000 miles roundtrip. Beginning in December, more than 20,000 gray whales (also known as California grays), make the slow but steady six-mile-per-hour swim from their feeding grounds in the Bering and Chuckchi seas north of Alaska to their breeding and birthing grounds in the lagoons of Baja.

“They’re not very fast, but they are very methodical and persistent,” said Lisa Gilfillan, education specialist, and the head of the whale watching program at Birch Aquarium at Scripps in La Jolla. “They travel hundreds of miles a day. It’s like a tortoise and the hare story. They do their best just to keep going.”

During this lengthy migration period, Birch Aquarium has partnered with Flagship Cruises and Events for the past 18 years to take marine mammal-loving guests on whale watching excursions to witness this spectacle. With 70 miles of coastline in their migration path, San Diego is considered one of the best locations to witness these gentle giants traveling south for the winter.

“Right now, we’re in the heart of the season where we’re seeing 10 to 20 whales per cruise,” said Gilfillan. “We usually see them breaching, spy-hopping or playing in the kelp forests at the point of Point Loma.”

But this is only one of two cruises Birch Aquarium offers to see these whales. In partnership with Andiamo Travel, aquarium members are also offered the chance to spend a week in Mexico to visit the lagoons where the whale calves are born. Gilfillan, who just returned from the lagoons last week, says most of the people on her trip became members just to go on this voyage.

“It’s a pretty magical experience,” said Gilfillan. “In the lagoons, the whales are very curious and will come right up to your boat and let you touch them. You will never forget that moment when you touch your first gray whale.”

Gray whales are considered to be “medium-sized wales,” only growing to be roughly 45-50 feet in length, according to the Marine Mammal Center. But once these animals get close, Gilfillan says there’s “no other feeling like it in the world.”

“It’s really humbling to see these massive animals up close and know that they’ve traveled thousands of miles to be in the same place that I happened to be in a week ago,” said Gilfillan.

However, interactions between gray whales and humans have not always been so peaceful. For hundreds of years, instead of being called “the friendlies,” they were called “devil fish.” According to Gilfillan, this was because the whales would “fight back” after being harpooned.

“For hundreds of years, they were hunted more so than other whales because gray whales hug the coast as they travel south,” said Gilfillan. “This means they are very easy to find and were very easy to hunt. Other whales dive deeper and go further offshore.”

The California grays rebounded from extinction not once, but twice, and actions to protect the species started in the 1940s, when the International Whaling Commission made it illegal to hunt the whales. In the 1970s, the Marine Mammal Protection Act and the Endangered Species Act made the gray whales amply more protected and the population is currently estimated at about 26,000, according to NOAA’s Southwest Fisheries Science Center in La Jolla.

“To think that now we have a relationship with them now where we’ve both learned to trust each other again and that we can go into their little sanctuaries in Mexico and you can see the moms bring their babies to the boat,” said Gilfillan. “I think that’s really magical, that two species who don’t speak the same language can have this amazing, personal interaction with each other. When you’re looking at them, they’re looking at you just as much.”

Morning tours run from 9:30 a.m. to 1:15 p.m., and afternoon tours run from 1:30 to 5 p.m. mid-December to late April every day.

Tours depart from Flagship Cruises & Events at the Broadway Pier, 990 N. Harbor Drive, downtown San Diego. Weekday admission is $42 adults and $21 children 4 to 12. Weekend admission is $47 adults and $25 children 4 to 12. Children 3 and under are free. For more information, visit aquarium.ucsd.edu or flagshipsd.com.